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<title>SSL/TLS Strong Encryption: How-To - Apache HTTP Server</title>
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<p class="menu"><a href="../mod/">Modules</a> | <a href="../mod/directives.html">Directives</a> | <a href="../faq/">FAQ</a> | <a href="../glossary.html">Glossary</a> | <a href="../sitemap.html">Sitemap</a></p>
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<p class="apache">Apache HTTP Server Version 2.2</p>
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<a href="http://www.apache.org/">Apache</a> > <a href="http://httpd.apache.org/">HTTP Server</a> > <a href="http://httpd.apache.org/docs/">Documentation</a> > <a href="../">Version 2.2</a> > <a href="./">SSL/TLS</a></div><div id="page-content"><div id="preamble"><h1>SSL/TLS Strong Encryption: How-To</h1>
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<p><span>Available Languages: </span><a href="../en/ssl/ssl_howto.html" title="English"> en </a></p>
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<p>The solution to this problem is trivial
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and is left as an exercise for the reader.</p>
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<p class="cite">-- <cite>Standard textbook cookie</cite></p>
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<p>How to solve particular security problems for an SSL-aware
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webserver is not always obvious because of the interactions between SSL,
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HTTP and Apache's way of processing requests. This chapter gives
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instructions on how to solve some typical situations. Treat it as a first
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step to find out the final solution, but always try to understand the
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stuff before you use it. Nothing is worse than using a security solution
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without knowing its restrictions and how it interacts with other systems.</p>
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<div id="quickview"><ul id="toc"><li><img alt="" src="../images/down.gif" /> <a href="#ciphersuites">Cipher Suites and Enforcing Strong Security</a></li>
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<li><img alt="" src="../images/down.gif" /> <a href="#accesscontrol">Client Authentication and Access Control</a></li>
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<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
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<h2><a name="ciphersuites" id="ciphersuites">Cipher Suites and Enforcing Strong Security</a></h2>
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<li><a href="#realssl">How can I create a real SSLv2-only server?</a></li>
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<li><a href="#onlystrong">How can I create an SSL server which accepts strong encryption only?</a></li>
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<li><a href="#upgradeenc">How can I create an SSL server which accepts strong encryption only, but allows
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export browsers to upgrade to stronger encryption?</a></li>
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<li><a href="#strongurl">How can I create an SSL server which accepts all types of ciphers in general, but
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requires a strong cipher for access to a particular URL?</a></li>
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<h3><a name="realssl" id="realssl">How can I create a real SSLv2-only server?</a></h3>
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<p>The following creates an SSL server which speaks only the SSLv2 protocol and
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<div class="example"><h3>httpd.conf</h3><p><code>
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SSLProtocol -all +SSLv2<br />
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SSLCipherSuite SSLv2:+HIGH:+MEDIUM:+LOW:+EXP<br />
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<h3><a name="onlystrong" id="onlystrong">How can I create an SSL server which accepts strong encryption
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<p>The following enables only the seven strongest ciphers:</p>
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<div class="example"><h3>httpd.conf</h3><p><code>
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SSLCipherSuite HIGH:MEDIUM<br />
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<h3><a name="upgradeenc" id="upgradeenc">How can I create an SSL server which accepts strong encryption
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only, but allows export browsers to upgrade to stronger encryption?</a></h3>
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<p>This facility is called Server Gated Cryptography (SGC) and requires
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a Global ID server certificate, signed by a special CA certificate
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from Verisign. This enables strong encryption in 'export' versions of
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browsers, which traditionally could not support it (because of US export
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<p>When a browser connects with an export cipher, the server sends its Global
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ID certificate. The browser verifies this, and can then upgrade its
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cipher suite before any HTTP communication takes place. The problem
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lies in allowing browsers to upgrade in this fashion, but still requiring
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strong encryption. In other words, we want browsers to either start a
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connection with strong encryption, or to start with export ciphers but
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upgrade to strong encryption before beginning HTTP communication.</p>
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<p>This can be done as follows:</p>
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<div class="example"><h3>httpd.conf</h3><p><code>
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# allow all ciphers for the initial handshake,<br />
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# so export browsers can upgrade via SGC facility<br />
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SSLCipherSuite ALL:!ADH:RC4+RSA:+HIGH:+MEDIUM:+LOW:+SSLv2:+EXP:+eNULL<br />
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<Directory /usr/local/apache2/htdocs><br />
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# but finally deny all browsers which haven't upgraded<br />
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SSLRequire %{SSL_CIPHER_USEKEYSIZE} >= 128<br />
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<h3><a name="strongurl" id="strongurl">How can I create an SSL server which accepts all types of ciphers
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in general, but requires a strong ciphers for access to a particular
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<p>Obviously, a server-wide <code class="directive"><a href="../mod/mod_ssl.html#sslciphersuite">SSLCipherSuite</a></code> which restricts
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ciphers to the strong variants, isn't the answer here. However,
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<code class="module"><a href="../mod/mod_ssl.html">mod_ssl</a></code> can be reconfigured within <code>Location</code>
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blocks, to give a per-directory solution, and can automatically force
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a renegotiation of the SSL parameters to meet the new configuration.
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This can be done as follows:</p>
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<div class="example"><p><code>
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# be liberal in general<br />
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SSLCipherSuite ALL:!ADH:RC4+RSA:+HIGH:+MEDIUM:+LOW:+SSLv2:+EXP:+eNULL<br />
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<Location /strong/area><br />
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# but https://hostname/strong/area/ and below<br />
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# requires strong ciphers<br />
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SSLCipherSuite HIGH:MEDIUM<br />
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</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
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<div class="section">
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<h2><a name="accesscontrol" id="accesscontrol">Client Authentication and Access Control</a></h2>
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<li><a href="#allclients">How can I force clients to authenticate using certificates?</a></li>
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<li><a href="#arbitraryclients">How can I force clients to authenticate using certificates for a
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particular URL, but still allow arbitrary clients to access the rest of the server?</a></li>
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<li><a href="#certauthenticate">How can I allow only clients who have certificates to access a
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particular URL, but allow all clients to access the rest of the server?</a></li>
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<li><a href="#intranet">How can I require HTTPS with strong ciphers, and either
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basic authentication or client certificates, for access to part of the
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Intranet website, for clients coming from the Internet?</a></li>
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<h3><a name="allclients" id="allclients">How can I force clients to authenticate using certificates?</a></h3>
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<p>When you know all of your users (eg, as is often the case on a corporate
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Intranet), you can require plain certificate authentication. All you
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need to do is to create client certificates signed by your own CA
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certificate (<code>ca.crt</code>) and then verify the clients against this
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<div class="example"><h3>httpd.conf</h3><p><code>
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# require a client certificate which has to be directly<br />
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# signed by our CA certificate in ca.crt<br />
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SSLVerifyClient require<br />
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SSLVerifyDepth 1<br />
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SSLCACertificateFile conf/ssl.crt/ca.crt
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<h3><a name="arbitraryclients" id="arbitraryclients">How can I force clients to authenticate using certificates for a
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particular URL, but still allow arbitrary clients to access the rest of the server?</a></h3>
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<p>To force clients to authenticate using certificates for a particular URL,
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you can use the per-directory reconfiguration features of <code class="module"><a href="../mod/mod_ssl.html">mod_ssl</a></code>:</p>
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<div class="example"><h3>httpd.conf</h3><p><code>
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SSLVerifyClient none<br />
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SSLCACertificateFile conf/ssl.crt/ca.crt<br />
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<Location /secure/area><br />
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SSLVerifyClient require<br />
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SSLVerifyDepth 1<br />
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</Location><br />
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<h3><a name="certauthenticate" id="certauthenticate">How can I allow only clients who have certificates to access a
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particular URL, but allow all clients to access the rest of the server?</a></h3>
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<p>The key to doing this is checking that part of the client certificate
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matches what you expect. Usually this means checking all or part of the
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Distinguished Name (DN), to see if it contains some known string.
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There are two ways to do this, using either <code class="module"><a href="../mod/mod_auth_basic.html">mod_auth_basic</a></code> or
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<code class="directive"><a href="../mod/mod_ssl.html#sslrequire">SSLRequire</a></code>.</p>
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<p>The <code class="module"><a href="../mod/mod_auth_basic.html">mod_auth_basic</a></code> method is generally required when
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the certificates are completely arbitrary, or when their DNs have
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no common fields (usually the organisation, etc.). In this case,
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you should establish a password database containing <em>all</em>
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clients allowed, as follows:</p>
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<div class="example"><h3>httpd.conf</h3><pre>
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<Directory /usr/local/apache2/htdocs/secure/area>
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SSLVerifyClient require
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SSLCACertificateFile conf/ssl.crt/ca.crt
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SSLCACertificatePath conf/ssl.crt
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SSLOptions +FakeBasicAuth
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AuthName "Snake Oil Authentication"
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AuthBasicProvider file
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AuthUserFile /usr/local/apache2/conf/httpd.passwd
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</Directory></pre></div>
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<div class="example"><h3>httpd.passwd</h3><pre>
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/C=DE/L=Munich/O=Snake Oil, Ltd./OU=Staff/CN=Foo:xxj31ZMTZzkVA
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/C=US/L=S.F./O=Snake Oil, Ltd./OU=CA/CN=Bar:xxj31ZMTZzkVA
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/C=US/L=L.A./O=Snake Oil, Ltd./OU=Dev/CN=Quux:xxj31ZMTZzkVA</pre></div>
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<p>When your clients are all part of a common hierarchy, which is encoded
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into the DN, you can match them more easily using <code class="directive"><a href="../mod/mod_ssl.html#sslrequire">SSLRequire</a></code>, as follows:</p>
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<div class="example"><h3>httpd.conf</h3><pre>
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<Directory /usr/local/apache2/htdocs/secure/area>
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SSLVerifyClient require
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SSLCACertificateFile conf/ssl.crt/ca.crt
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SSLCACertificatePath conf/ssl.crt
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SSLOptions +FakeBasicAuth
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SSLRequire %{SSL_CLIENT_S_DN_O} eq "Snake Oil, Ltd." \
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and %{SSL_CLIENT_S_DN_OU} in {"Staff", "CA", "Dev"}
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</Directory></pre></div>
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<h3><a name="intranet" id="intranet">How can I require HTTPS with strong ciphers, and either basic
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authentication or client certificates, for access to part of the
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Intranet website, for clients coming from the Internet? I still want to allow
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plain HTTP access for clients on the Intranet.</a></h3>
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<p>These examples presume that clients on the Intranet have IPs in the range
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192.160.1.0/24, and that the part of the Intranet website you want to allow
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internet access to is <code>/usr/local/apache2/htdocs/subarea</code>.
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This configuration should remain outside of your HTTPS virtual host, so
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that it applies to both HTTPS and HTTP.</p>
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<div class="example"><h3>httpd.conf</h3><pre>
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SSLCACertificateFile conf/ssl.crt/company-ca.crt
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<Directory /usr/local/apache2/htdocs>
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# Outside the subarea only Intranet access is granted
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Allow from 192.168.1.0/24
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<Directory /usr/local/apache2/htdocs/subarea>
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# Inside the subarea any Intranet access is allowed
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# but from the Internet only HTTPS + Strong-Cipher + Password
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# or the alternative HTTPS + Strong-Cipher + Client-Certificate
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# If HTTPS is used, make sure a strong cipher is used.
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# Additionally allow client certs as alternative to basic auth.
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SSLVerifyClient optional
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SSLOptions +FakeBasicAuth +StrictRequire
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SSLRequire %{SSL_CIPHER_USEKEYSIZE} >= 128
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# Force clients from the Internet to use HTTPS
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RewriteCond %{REMOTE_ADDR} !^192\.168\.1\.[0-9]+$
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RewriteCond %{HTTPS} !=on
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# Allow Network Access and/or Basic Auth
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# Network Access Control
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# HTTP Basic Authentication
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AuthName "Protected Intranet Area"
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AuthBasicProvider file
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AuthUserFile conf/protected.passwd
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</Directory></pre></div>
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<p><span>Available Languages: </span><a href="../en/ssl/ssl_howto.html" title="English"> en </a></p>
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<p class="apache">Copyright 2006 The Apache Software Foundation.<br />Licensed under the <a href="http://www.apache.org/licenses/LICENSE-2.0">Apache License, Version 2.0</a>.</p>
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